1. Field of the Invention
The present invention relates to a detector which utilizes noise generated by a magnetoresistive (MR) element adjacent a magnetic tape for locating an edge of the magnetic tape.
2. Description of the Related Art
A magnetic tape drive employs a magnetic head to write and read data with respect to one or more tracks on a magnetic tape. The edge of the magnetic tape is employed as a reference for establishing or finding a track, whether the track be a data track or a servo track. In the prior art, a typical multi-track magnetic tape is a quarter-inch tape with eight tracks on it. The edge of the tape is positioned within the drive by a guide which occupies a precisely known position with respect to the location of the magnetic head. When the tape is placed in the drive, the tape edge is positioned by the guide and the head is moved a predetermined distance from a known position onto the tape to selected track positions. This arrangement works well for relatively low track densities, such as 32 tracks per inch (TPI). However, it is insufficiently precise to consistently and accurately locate a small head (3-4 .mu.m) with respect to a very narrow track in a high TPI environment.
In order to locate information and servo tracks with repeatable accuracy when track density is high, numerous new arrangements have been proposed for accurately determining the edge of a magnetic tape. Most of these proposed arrangements employ a write transducer to write a control track along an edge of the tape, a read transducer to read signals in the control track, and a device for comparing the signal read with a predetermined reference signal to determine the edge of the tape. The following U.S. Pat. Nos. embody this basic technique: 5,111,347; 4,977,468; 4,611,249; 4,476,503; 4,786,984; and 4,802,030. In the last U.S. Pat. No., 4,802,030, the noise read by the transducer is considered to be a problem and the patent teaches establishing and detecting a reference signal which has an amplitude greater than the noise for finding the edge of the tape. As will become clear hereinafter, the present invention does not treat the noise picked up by the read transducer as a problem, but instead utilizes the noise as a solution for detecting the edge of the magnetic tape.
A narrow track read transducer typically employs a magnetoresistive (MR) element sandwiched between thin film gap and shield layers. In operation, a sense current is transmitted through the MR element, with appropriate bias, to establish the element's quiescent state. When the read transducer engages a moving magnetic tape, magnetic fields from the tape vary the sense current and resistance of the MR element. This variation generates the readback signal and is detected by a differential amplifier connected across the MR element. The MR element also produces a slight amount of inherent noise when it is off the tape. When the MR element is on the tape, friction between the MR element and the moving magnetic tape increases the temperature of the MR element, causing the MR element to produce a marked increase in its noise level. While the prior art has always considered the off tape and on tape noise to be undesirable, it is actually beneficially employed by the present invention to find the edge of the magnetic tape.